ISSA Explanatory Supplement
IV. ANALYSIS RESULTS
B. Positional Accuracy

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Positional accuracy and small source photometric consistency were studied using point sources selected from the IRAS Point Source Catalog to be bright enough to measure easily in the ISSA data and to be free of interference from extended emission or nearby point sources. Point sources at 12 and 25 µm were selected to be at high Galactic latitude, |b|>30°, with brightness between 5 and 55 Jy and correlation coefficients of 0.99 or greater. The 23 sources at 12 µm and 24 sources at 25 µm are listed in Table IV.B.1. Point sources at 60 and 100 µm were selected from the IRAS galaxy list (Soifer et al., 1987) to have a large radial velocity (>4000 km s-1) with brightness between 5 and 55 Jy. The velocity criterion is designed to select small angular diameter galaxies unlikely to be extended at the resolution of the ISSA maps. The 14 sources at 60 µm and 15 sources at 100 µm are listed in Table IV.B.1.

Table IV.B.1 Point Sources Used in the Position and Photometric Analyses
12 µm25 µm 60 µm100 µm
IRAS NameField IRAS NameField IRAS NameField IRAS NameField
00515-6308 25 00515-6308 25 00163-1039 162* 00163-1039 162*
01452-8026 3 01452-8026 3 02228-2500 {1}98 02228-2500 {1}98
02039-5722 26 02039-5722 26 04315-0840 169* 04315-0840 169*
02110-7143 11 02110-7143 11 04461-0624 169* 04461-0624 169*
02238-5947 26 02238-5947 26 05189-2524 135 05189-2524 135
03040-8013 3 03040-8013 3 08354+2555 316* 09320+6134 397
03336-7636 3 03336-7636 3 09320+6134 397 09437+0317 213*
05150-4056 73 04330-6307 28 10565+2448 319* 10565+2448 319*
05174-3345 102 05150-4056 73 12540+5708 399 12540+5708 399
05217-3943 73 05174-3345 102 12590+2934 322* 12590+2934 322*
05345-4406 73 05217-3943 73 13136+6223 399 13136+6223 399
10416+6740 414 05345-4406 73 13136+6223 400 13136+6223 400
10521+7208 414 10416+6740 414 13183+3423 322* 13183+3423 322*
11284+6936 415 10521+7208 414 23488+2018 270* 23488+2018 270*
15448+3828 355 11284+6936 415 23591+2312 270* 23591+2312 270*
15566+3609 355 15448+3828 355
16164+5952 402 15566+3609 355
16342+6034 402 16164+5952 402
17126+3625 357 16342+6034 402
17133+3651 357 17126+3625 357
17329+5359 383 17133+3651 357
17403+6234 403 17329+5359 383
17403+6234 403
20427-8243 9
*These fields are from the || < 50° sky.

Figure IV.B.1(a) Histograms of Position Differences in RA Between PSC and ISSA
larger largest

The position accuracy analysis consisted of comparing the apparent position of the selected sources in the ISSA maps with the published source locations in the IRAS Point Source Catalog. The ISSA position was taken to be the photocenter of the source in the ISSA map. Photocenters were obtained by producing a flux-weighted average of the pixel positions over a circular region surrounding a source after subtraction of a local background. Figures IV.B.1(a) and IV.B.1(b) display histograms of difference between the PSC position and the ISSA position for the selected point sources. Table IV.B.2 summarizes the statistics obtained from the histograms. The mean position differences between the PSC and ISSA are less than 0.1 pixel (9") and the expected position uncertainty of a single source is less than 0.2 pixel (18").

Figure IV.B.1(b) Histograms of Position Differences in DEC Between PSC and ISSA
larger largest

Table IV.B.2 ISSA and ISSA Reject Fields Affected by Saturated Data
12 µm25 µm 60 µm100 µm
# of Sources 23 24 14 15
Mean Delta R.A. (') 0.004 0.001 0.040 0.031
sigma(Delta R.A.) (') 0.156 0.267 0.179 0.224
Mean Delta Dec. (') 0.070 0.034 -0.078 -0.131
sigma(Delta Dec.) (') 0.126 0.225 0.156 0.236

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